1. Technical Field
The present invention relates to method and apparatus as well as a corrective optical system respectively for evaluating a restoration-premised lens and, specifically, the invention relates to such method and apparatus as well as a corrective optical system respectively for evaluating a lens which is premised on a restoration processing for extending the depth of focus of this lens. Referring further to this lens, owing to execution of such restoration processing, the lens is designed such that it is forced not to form an image and, specifically, in a high frequency range, the depth of the MTF (Modulation Transfer Function) thereof is decreased to thereby provide an image as if it is blurred; and, the lens is actually manufactured according to this design.
2. Related Art
Recently, there has been remarkably advanced a technology which converts an optical image obtained by an optical system to an electronic image, imports the electronic image as image data, and enforces an image processing on the image data to thereby obtain a desired image. With use of this technology, even when a blur image is obtained by an optical system having a poor image quality, the image processing can be executed to the blur image and finally a sharp image can be obtained from the blur image. Thus, there has been developed an optical system which is premised on such restoration processing that can provide a sharp image.
In non-patent reference 1 (W. Thomas Cathey and Edward R. Dowski, “New paradigm for imaging systems”, APPLIED OPTICS/Vol. 41, No. 29/10 Oct. 2002) and non-patent reference 2 (Edward R. Dowski, Jr., and W. Thomas Cathey, “Extended depth of field through wave-front coding”, APPLIED OPTICS/Vol. 34, No. 11/10 Apr. 1995) as well as in the below-mentioned patent reference 1 (WO-A2-2004/090581), patent reference 2 (JP-B-3275010 corresponding to WO-A1-96/24085), patent reference 3 (JP-A-2000-275582), patent reference 4 (JP-A-2007-60647) and patent reference 5 (JP-A-2006-519527), there is disclosed a system which forms an image using the above-mentioned technology. FIG. 8 shows the general structure of this system 20. In the system 20 shown in FIG. 8, the image 23 of an object 21 is formed by an optical system 22. This optical system 22 is structured such that, differently from a conventional imaging optical system, forms an blur image having a given amount of blur even at the image forming position of a so called conventional optical system; and thus, the image provides a blurred image. That is, the optical system 22 is structured such that, as will be described later, it forms an image premised on such restoration processing that is executed by an image processing portion 25. A lens system having such property as the optical system 22 will be hereinafter referred to as a lens premised on restoration processing (which is also abbreviated to a restoration-premised lens).
An image pick up element 24 picks up this blurred image 23, converts it to an electric signal and transmits the thus-converted electric signal to the image processing portion 25. The image processing portion 25 enforces such a restoration processing as reconstructs the blurred image into an image having the intensity distribution of an image in focus using a digital filter, thereby forming a sharp image 26.
In the non-patent reference 1 and 2 and the patent reference 1 to 4, there is disclosed a field depth extending system which extends the depth of field by applying the above system 20; and, a lens premised on a restoration processing to be enforced by this system 20 is designed such that it extends its depth of field.
In the non-patent reference 1 and 2 and patent reference 2 to 4, there is disclosed a lens system into which, as a lens premised on the above restoration processing, a mask for demodulating a phase distribution is inserted to the pupil position of a conventional imaging lens (which is hereinafter referred to as a pupil demodulation type mask).
On the other hand, in the patent reference 5, there is disclosed a lens system which does not employ the pupil demodulation type mask as a lens premised on the restoration processing but generates such an aberration on purpose that forms a blurred image (which is hereinafter referred to as an aberration additional type lens system).
However, in a pupil modulation type image system disclosed in the non-patent reference 1, since there is used a mask having a rotation asymmetry with respect to an optical axis, there is found a disadvantage that the performance of the system can be varied sensitively with respect to the rotation around the optical axis. Owing to this, in order to obtain a sharp image, the position of the mask with respect to the rotation around the optical axis must be strictly matched to the direction of an image processing which corresponds to the mask. Also, in the pupil demodulation type lens system, due to the increased number of parts, unfavorably, the cost of the lens system increases.
On the other hand, the aberration additional type lens system does not require the use of a mask and also can be structured such that it is has rotation symmetry with respect to an optical axis. From these viewpoints, this lens system is preferable. However, in the case of the aberration additional type lens system, even when it can be manufactured as designed, there can be obtained only blurred images at all positions. This makes it difficult to regard it as an imaging optical system which is capable of specifying an image forming position. Therefore, in the evaluating step thereof, it cannot employ evaluating method and apparatus which are based on the image forming position that is used in the evaluation of a conventional imaging optical system.
To evaluate the aberration additional type lens system, using such structure as shown in FIG. 8, a blurred image must be once converted to an electric signal to thereby obtain image data, an image processing must be enforced on the thus obtained image data, and the image after execution of the image processing must be evaluated. However, this method takes much time and cost and, therefore, this method is not able to cope with the mass production of the lens systems. Also, when the image processing methods are different, there is generated a difference between images obtained after execution of the image processing, which makes it impossible to evaluate the performance of only the optical systems.
The present invention aims at solving the problems found in the above-mentioned related art. Thus, it is an object to provide restoration-premised lens evaluating method and apparatus which can evaluate the performance of only an optical system including even an aberration additional type of restoration-premised lens using a general-purpose technique similarly to a conventional imaging optical system, and an evaluating corrective optical system for use such evaluation.